This invention relates generally to techniques for selectively detecting photons and, more specifically, for detecting a weak omnidirectional and monochromatic photon signal in the presence of strong broadband background radiation, such as sunlight. There are many applications, such as laser communication receivers, environmental filters, and scatterometers, in which it is necessary to detect such a narrowband photon signal, perhaps resulting from a scattered laser pulse. Although very narrowband optical filters can be made using interferometric techniques, the photon acceptance angles of such filters are necessarily quite small, making them unsuitable for many applications.
One approach to narrow bandwidth filtering with wide acceptance angles is the use of atomic absorption/fluorescence filters, such as are described in a paper by Marling et al J. Appl. Phys. 50, 610 (1979). Atomic absorption/fluorescence narrow bandwidth infrared up-converters are described in a paper by Gelbachs et al., IEEE Trans. on Electron Devices, VOL ED-27, 99 (1980).
Two other areas of subject matter are also material to the field of the present invention, but, as will shortly be appreciated, are clearly distinguishable from the invention as defined in the claims. One area is opto-galvanic detection, and the other is isotopic separation by selective photoionization.
In opto-galvanic detection of optical absorptions, photons are absorbed by atoms and result in a transition to a bound, metastable, excited state of the atoms. Then the metastable-state atoms are ionized by the impact of electrons within a plasma. There are a number of difficulties with applying this technique to the detection of low light levels over narrow bandwidths. First, photons of different energies will excite atoms to different metastable states, all of which will be ionized, non-selectively, by electron impact within the plasma. Therefore, without careful optical prefiltering of the photons the opto-galvanic effect cannot discriminate between different photon energies, which correspond to different wavelengths. Another difficulty is that opto-galvanic filters are not effective in detecting low light levels. The opto-galvanic effect depends on a change in the plasma conditions caused by optical absorption. This generally requires illumination with a strong light source, such as a laser, so that enough new charge carriers are produced to change the measurable electrical impedance of the plasma. Other disadvantages are that opto-galvanic devices cannot be operated in a pulsed mode, and that the minimum achievable filter bandwidth is not narrow enough for some applications.
Examples of patents describing the optogalvanic effect are U.S. Pat. Nos. 4,148,586 and 4,184,127, both issued to Green et al. and 4,402,606 to Zalewski et al.
The process of isotopic separation by selective photoionization is described in U.S. Pat. Nos. 3,443,087 to Robieux et al., 3,772,519 to Levy et al. 3,937,956 to Lyon, and 4,085,332 to Fletcher et al. Isotopes, which have similar or identical chemical properties but different atomic weights, are difficult to separate by chemical means. In the photoionization process, photon beams are used to selectively ionize one isotope in a mixture of isotopes, and the resulting ions are separated out by application of an electric or magnetic field. To operate effectively, the separation technique requires that practically all of the atoms of the selected isotope will absorb photons and be transformed to a metastable or excited state, which can then be subsequently ionized and separated.
Another technical area of interest is that of optical filters based on resonance fluorescence or resonance scattering. In filters of this type, an incoming photon is absorbed in an atomic or molecular transition to an excited state. The atom or molecule in the excited state then reradiates a photon at the same or a lower frequency. A major disadvantage of this approach is that the reradiated photon must be collected and detected, or converted into an electron for detection, processes that usually decrease the efficiency of the filter.
It will be appreciated from the foregoing discussion that there is still a need for an optical filter that has an extremely narrow pass bandwidth, a wide photon acceptance angle, and the ability to detect small photon signals with high quantum efficiency in the presence of a broadband background level of radiation. The present invention is directed to this end.